1. Field of the Invention
The invention relates to methods for manufacturing a large-size bend-insensitive low-water-peak single mode optical fiber preform and an optical fiber.
2. Description of the Related Art
In the process of manufacturing fibers, because of the existence of the absorption peak (it is also known as “water peak”) caused by the hydroxyl (OH) within 1360-1460 nm, the usage of the fibers at the wavelength range is limited. To apply the fibers in the whole wavelength range, the water peak within that range shall be eliminated. Thus, the fibers can offer an available wavelength range with a width as wide as 400 nm. In accordance with the specification of ITU-T G.652.C/D, the fibers, having the attenuation less than the specified value of 1310 nm within the range of 1383±3 nm, are generally called “low water peak fibers” or “zero water peak fibers”. In ITU-T G.652.C/D optical fiber standard, it has been clearly stipulated that the attenuation coefficient of optical fibers at the 1383±3 nm wavelength band is not more than 0.4 dB/km. Such optical fibers are combined with the CWDM (coarse wavelength-division multiplexing) technology, thus the larger information capacity and longer distance without repeater can be provided.
In recent years, with the development of the optical fiber access network, the laying of optical fibers is getting closer to the end user. When considering that optical fibers are laid in buildings or indoors, optical fibers should have good bending resistance including optical bending resistance and mechanical bending resistance. During the process of laying optical fibers in buildings or indoors, when the bending radius of optical fibers is 10 mm, 7.5 mm, or even 5 mm, optical fibers must have the performance of low additional loss under the condition of extremely small bending radius. In a miniature optical device, it also requires that optical fiber has low additional loss under the small bending radius, so as to reduce the space occupied by optical fibers. Meanwhile, the mechanical properties of optical fibers are required to be enhanced, so as to ensure the mechanical reliability of optical fibers in the long-term small bending radius working state.
Since the bending radius of conventional fibers with a low water peak (in conformity with ITU-T G.652C/D) is generally 30 mm, laying such fibers indoors or in narrow spaces is greatly restricted, especially the ones with long wavelength (U wave wavelength band: 1625-1675 nm). For this reason, it is required to design and develop a fiber with bending insensitive properties to satisfy the FTTH installation and the usage requirements of long wavelength. In December 2006, ITU-T came up with a new fiber standard (G.657 fiber): Characteristics of a bending loss insensitive single mode optical fiber and cable for the access network. Thus, developing single-mode fibers with a low water peak and anti-bending properties is of great significance for promoting the development of the FTTx technology.
In the present bend-insensitive single mode optical fibers with obviously improved bending properties, the purpose of reducing the bending loss is achieved mainly through designing a waveguide structure different from G.652 optical fibers.
Studies show that the bending resistance of optical fibers can be enhanced by adopting the structural design of an air cladding layer, but the optical fiber is relatively high in manufacturing cost, complex in process, difficult in optical-fiber connection and not favorable for popularization and application.
Through the design of a depressed cladding layer, the bending resistance of optical fibers can be effectively enhanced, the bend-insensitive single mode optical fibers can be realized, the additional loss of optical fibers under the small bending radius can be effectively reduced, however, the mechanical reliability of optical fibers under the small bending radius is decreased.
When a fiber bends, the outside thereof is exposed to the tensile stress. The tensile stress is represented by the following formula:
  σ  =            E      ·      r              (              R        +                  C          th                +        r            )      
wherein E represents young modulus of quartz glass, R represents a bending radius, r represents the radius of a fiber, and Cth represents the thickness of a coating. Based on the formula and the bending radius, the tensile stress imposed on a fiber with a glass cladding diameter of 125 μm and an outer diameter of 250 μm is calculated, as shown in FIG. 6. For example, when the bending radius is decreased to 6.5 mm, the tensile stress imposed on the outer bending wall of the fiber is 0.69 GPa (100 kpsi), which reaches the common screening tension of fibers. Bending easily causes fracture, thereby increasing the building and maintenance cost and affects the reliability of systems. The appendix of fiber standard ITU-T G.657 briefly describes the prediction of fiber life. The service life of fibers is related to the dynamic stress corrosion susceptibility parameter thereof. Under identical bending radius and storage length, the higher the dynamic stress corrosion susceptibility parameter of fibers, the higher the mechanical reliability thereof. Thus, it is urgent to develop a fiber that has low additional bending loss and stable mechanical properties.
There are four conventional methods to manufacture a fiber preform: modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD), outside vapor deposition (OVD), and vapor axial deposition (VAD). The MCVD and PCVD methods belong to an inner tube method, and thus, if an outer depressed cladding layer is required, it is difficult to make a large-sized preform (with a diameter over 100 mm) due to the limit of the tubes. Furthermore, the inner tube method has a low deposition rate. When OVD and VAD methods are applied, it is required to make a fluorine-doped cladding layer in the process of depositing a core layer and an inner cladding layer. However, the process is difficult to control and the refractive index profile cannot be effectively controlled due to dispersion of fluorine during the sintering process. A practical production method is to first deposit a core rod including a cladding layer with a certain thickness, followed by dehydration and sintering, and then to deposit a fluorine-doped cladding layer on the glass core rod. The fluorine can be directly added during the deposition process or during the sintering process. As the OVD and VAD methods both belong to a flame (H2/O2) hydrolysis method, the deposits have to be directly exposed to the hydrogen/oxygen flame (H2/O2) when deposition occurs on the glass core rod. Thus, a large amount of hydroxyl (OH) produced from the H2/O2 flame will spread into the core layer, resulting in an increase in the water peak attenuation of the fibers; therefore, the cladding layer around the glass core rod shall be thick enough to prevent the OH from spreading inwards. However, if the cladding layer is too thick, the fluorine-doped cladding will be far from the core layer, and therefore the anti-bending performance of the fibers cannot be improved.
In the conventional optical fiber preform process, low-water-peak optical fiber core rods with smaller size are required to be prepared first, if an inner tube method is adopted, a lining tube is required, and a lathe is required for melting contraction of core rods after the completion of deposition of core rods. Meanwhile, a small fluorine-doped quartz glass lining tube is prepared, then the optical fiber core rods are sleeved in the small fluorine-doped quartz glass casing tube, combined core rods are obtained through melting contraction, and finally, the combined core rods are externally clad to form large-size preforms for fiber drawing. In the technical scheme, a lining tube is required for depositing low-water-peak core rods, meanwhile, various treatment of melting contraction, corrosion, cleaning, and drying is carried out to the low-water-peak core rods, and melting contraction is performed to the core rods and small fluorine-doped quartz glass casing tube. The large-size preforms are formed after sleeving and melting contraction twice, thus not only the processing links are more, the process is more complex, but also the low-water-peak performance and bending resistance of the manufactured optical fibers are also affected due to the more sleeving interfaces.